Simulating the Formation of Secondary Organic Aerosol from the Photooxidation of Aromatic Hydrocarbons

Abstract

Environmental Context. Atmospheric particulate material can affect the radiative balance of the atmosphere and is believed to be detrimental to human health. Secondary organic aerosols (SOA), which make a significant contribution to the total atmospheric burden of fine particulate material, are formed in situ following the photochemical transformation of organic pollutants into relatively less-volatile, oxygenated compounds which can subsequently transfer from the gas phase to a particle phase. SOA formation from the atmospheric photooxidation of aromatic hydrocarbons—present, for example, as a result of automobile use—is believed to be important in the urban environment and yet the mechanisms are not well understood. For example, even the reasons for observed variations in the relative propensity for SOA formation, from the photooxidation of various simple aromatic hydrocarbons, are not clear. Abstract. The formation and composition of secondary organic aerosol (SOA) from the photooxidation of benzene, p-xylene, and 1,3,5-trimethylbenzene has been simulated using the Master Chemical Mechanism version 3.1 (MCM v3.1) coupled to a representation of the transfer of organic material from the gas to particle phase. The combined mechanism was tested against data obtained from a series of experiments conducted at the European Photoreactor (EUPHORE) outdoor smog chamber in Valencia, Spain. Simulated aerosol mass concentrations compared reasonably well with the measured SOA data only after absorptive partitioning coefficients were increased by a factor of between 5 and 30. The requirement of such scaling was interpreted in terms of the occurrence of unaccounted-for association reactions in the condensed organic phase leading to the production of relatively more nonvolatile species. Comparisons were made between the relative aerosol forming efficiencies of benzene, toluene, p-xylene, and 1,3,5-trimethylbenzene, and differences in the OH-initiated degradation mechanisms of these aromatic hydrocarbons. A strong, nonlinear relationship was observed between measured (reference) yields of SOA and (proportional) yields of unsaturated dicarbonyl aldehyde species resulting from ring-fragmenting pathways. This observation, and the results of the simulations, is strongly suggestive of the involvement of reactive aldehyde species in association reactions occurring in the aerosol phase, thus promoting SOA formation and growth. The effect of NOx concentrations on SOA formation efficiencies (and formation mechanisms) is discussed.